Optical communication systems can include information signals that are transmitted through an optical transmission medium. FIG. 1 shows an optical medium 110 (typically an optical fiber) that can serve as a transmission medium for optical signals. Generally, a transducer 120 converts electrical signals generated by a signal source 130 into optical signals. The electrical signals of the signal source 130 can be modulated by information 140. Therefore, the optical signals are modulated by the information 140. After transmission through the optical medium 110, the optical signals can be received by another transducer 150. The transducer 150 can convert the optical signals back to electrical signals.
Optical communication systems generally include transmission of multiple carrier signals in which each of the carrier signals is transmitted at a different transmit frequency. Each individual transmission signal is typically modulated by an information signal. Each of the transmission signals can be individually received, and the information signals can be detected.
FIG. 2 shows a frequency spectrum of multiple transmission signals. The transmission signals each include a carrier frequency FC1, FC2, FC3, FC4. The frequency spectrum allocated to each of the carrier frequencies is generally referred to as a transmission channel. The amount of frequency spectrum allocated to each transmission channel generally determines the amount of information that can be transmitted through the transmission channel. It is desirable to utilize as much of the allocated frequency spectrum as possible.
The frequency spectrum of FIG. 2 shows transmission signals 210, 220, 230, 240 at the carrier frequencies FC1, FC2, FC3, FC4. Frequency spectrum adjacent to each of the transmission signals 210, 220, 230, 240 is generally occupied by information that is modulated onto the transmission signals 210, 220, 230, 240. Generally, the greater the modulation rate of the information (typically, the modulation rate is proportional to the amount of information) the greater the amount of frequency spectrum occupied by each transmission signal and associated modulation information. The modulation rate of each transmission signal should not be so large that the modulation information of one transmission signal interferes with the modulation information of a neighboring transmission signal.
FIG. 3 shows a frequency spectrum of multiple transmission signals 310, 320, 330, 340 in which information from neighboring transmission channels overlap. That is, information intended for transmission through one transmission channel, is unintentionally transmitted within another transmission channel. For example, the modulation information of the first transmission signal 310 overlaps with the modulation information of the second transmission signal 320, as designated 315. The modulation information of the second transmission signal 320 overlaps with the modulation information of the third transmission signal 330, as designated 325. The modulation information of the third transmission signal 330 overlaps with the modulation information of the fourth transmission signal 340, as designated 335.
The overlap can be due to distortion of the transmission signals due to components within a transmission system being non-ideal. The distortion can include noise, spurious signals and harmonics of transmission signals overlapping with neighboring transmission signals.
Information signal channel frequency overlap from one transmission channel to another transmission channel, introduces transmission errors. Transmission errors reduce the effectiveness of a communication system. Additionally, transmission errors can reduce the transmission bandwidth of a communication.
It is desirable to provide filtering of optical communication signals to reduce the amount of frequency spectrum overlap between transmission signals of the communication signals.